The invention relates to a projection device comprising a projector for providing a light beam to be projected in polarized light, a means for changing the direction of polarization, and a transmissive projection screen, said means for changing the direction of polarization being arranged between the projector and the transmissive projection screen.
The invention also relates to a method of adjusting the diffusion of a projection screen in a projection device comprising a projector for providing a light beam to be projected in polarized light, a means for changing the direction of polarization, and a transmissive projection screen, said means for changing the direction of polarization being arranged between the projector and the transmissive projection screen.
Such a device and method are known from U.S. Pat. No. 5,694,245. This patent describes that the means for changing the direction of polarization causes the direction of projection of the light beam to rotate through a predetermined fixed angle.
A transmissive projection screen is intended for projecting an image from a position behind the screen and viewing the image from a position in front of the screen. Such screens comprise a diffusion layer for scattering the light projected by the projector from behind on the diffusion layer into more directions on the front side. To improve the visibility and brightness of the image on the screen, lenticular lens elements on the front side of the screen as well as black stripes have been used in the past. The diffusion of such screens, i.e. the quantity of light emitted in directions deviating from the normal on the screen is such that most light is directed towards the location where the designer of the projection device expects the viewers to be present, usually right in front of the screen. Consequently, the screen is built up in such a way that the brightest image can be viewed right in front of the screen. Another consequence is that, when the screen is viewed at an angle, the screen has a lower brightness than in the case where it is viewed right in front.
If much light is emitted straight to the front by the screen, this automatically results in a strong dependence of the viewing angle as regards the quantity of light that can be observed in other directions. There are people who regard such a viewing angle dependence as an unacceptable aspect and therefore do not purchase such a device, for example, a wide-screen television set.
It is an object of the invention to provide a projection device in which the above-mentioned drawbacks are mitigated.
To this end, a projection device according to the invention is characterized in that the transmissive projection screen comprises a polarizing diffusion layer, in that the extent of changing the direction of polarization of the means for changing the direction of polarization is adjustable, and in that adjusting means, coupled to the means for changing the direction of polarization, are provided for adjusting the extent of changing the direction of polarization.
It is thereby achieved that the extent of diffusion and hence the extent to which the light leaves the projection screen from each pixel into a plurality of directions is adjustable by adjusting the direction of polarization of the light beam to be projected. If, as is known from the prior art, a small extent of diffusion is desired, the direction of polarization of the light beam to be projected will be chosen to be such that the polarizing diffusion layer passes the light to a maximally unhindered extent. However, if a large range around the screen is desired with a minimal variation of brightness and as a function of the angle at which the image is viewed on the projection screen, the direction of polarization of the light beam to be projected should be adjusted by the adjustable means for changing the direction of polarization, such that there will be a great extent of diffusion in the polarizing diffusion layer.
Polarizing diffusion layers are known per se from, inter alia, U.S. Pat. No. 5,900,977 and the Japanese published patent application Ser. No. 07/333,428 and do not need any further description in this context.
A preferred embodiment of a projection device according to the invention is characterized in that the means for changing the direction of polarization comprises an adjustable liquid crystal element, in that the extent of changing the direction of polarization by the liquid crystal element is electrically adjustable, and in that the adjusting means are adapted to apply an adjustable electric field across the liquid crystal element.
Liquid crystals are suitable for changing the directions of polarization, and the extent of changing the direction of polarization can also be easily adjusted in liquid crystals by means of the intensity of an electric field across the liquid crystal element.
To this end, a method according to the invention is characterized in that a transmissive projection screen comprising a polarizing diffusion layer is used, in that an adjustable means for changing the direction of polarization is used, and in that the direction of polarization of the light beam to be projected is adjusted by the adjustable means for changing the direction of polarization.
It is thereby achieved that the viewing angle at which the projection screen can be viewed is adjustable in a simple way, namely by adjusting the means for changing the direction of polarization, without a too large extent of decreasing brightness.